During ground operations. i.e. taxiing, aircraft pilots rely solely on charts, taxi-way markings and illuminated marker boards to determine their current location and the direction to their next position. Complex and unfamiliar airport layouts and/or low visibility conditions can lead to pilots mistakenly taking a wrong direction, leading to potentially dangerous situations, for example a taxiing aircraft turning on to a runway in the path of another aircraft taking off or landing, or an aircraft attempting to take off from the wrong runway, perhaps one obstructed by repair works or other aircraft. A less dangerous consequence of unfamiliarity with ground layout, but one with potentially significant economic effects, is that landing aircraft may slow down excessively after touch down to enable the pilot to locate his turn off to the taxiway. This increases the time the aircraft occupies the main runway, delaying its availability for the next landing. Any reduction in the capacity of the airport to handle incoming and departing flights has a very significant cumulative cost, and even small reductions in runway occupancy can effect substantial savings.
It is therefore desirable to provide pilot with better information as to their position on the airport and guidance as to routing on the ground, especially in low visibility conditions, for example in bad weather or at night.
Most aircraft are currently equipped with an Instrument Landing System receiver to receive signals transmitted by marker beacon transmitters during the approach to the runway. The Marker frequency is 75 MHz, and aircraft systems are arranged to receive one of three audio frequency tones distinguishing the approach markers, although now only two markers are generally provided, an outer marker at 400 Hz at four miles from the runway and a middle marker at 1.3 kHz at one mile from the runway, each with a distinctive audible modulation pattern. Receipt of the different tones causes a different coloured light to illuminate, i.e. blue for the outer marker and amber for the middle marker, to give the pilot an audible and visible indication of his distance from the runway. When the aircraft is on the ground, the Marker receiver is redundant. Further, while the gradual phasing out of ILS marker beacons in favour of more sophisticated low visibility systems is beginning to take place, it will be some considerable time before the airborne equipment is no longer required.
It has been proposed to use the ILS receiver in a system to provide ground guidance information. U.S. Pat. No. 5,689,273 discloses a system for guiding an aircraft by providing a pair of inductive loops disposed around each side of a path to be followed by the aircraft. An inductive sensor on the aircraft senses the composite magnetic field induced by the loops and produces an RF signal which is fed to the ILS system to give an indication of the deviation from the centre line of the path. A further feature provides transmitters giving marker radio beams across taxiways/runways transmitting different tones to indicate the type of path in conjunction with the coloured light of the ILS system. While this system might enable the pilot to avoid confusion between a taxiway and a runway, for example, it cannot provide any clear guidance as to whether the pilot is heading in the correct direction on the correct taxiway or runway.